1. Technical Field
The present invention relates to an AV signal receiving circuit which transmits and receives signals containing audio and video information in high quality, and an audio and video receiving apparatus containing the same.
2. Description of Related Art
Recently, as AV equipment such as DVD recorders and televisions have advanced, there have been demands for technology for transmitting high definition baseband audio/video signals using a high-speed interface. Exemplary standards among high-speed interfaces are DVI (Digital Visual Interface) and HDMI (High-Definition Multimedia Interface). DVI is a standard used for digital transmission of video signals to a LCD or a CRT. HDMI is a digital interface standard for next-generation television which has additional functions for home-use electric equipment and downward compatibility based on DVI. These standards can be used as high-speed interfaces.
However, high frequency audio/video signals are used in cases where the audio/video signal is transmitted for connection between LSIs or between devices at high speed by using these high-speed interfaces. Accordingly, the distances that signals pass through a signal transmission circuit, a signal receiving circuit and a transmission line cannot be ignored. Accordingly, it is necessary to introduce the concept of distance and to take the impedance of transmission line for high frequency signals into consideration. When the impedance of a signal source does not match with the impedance of a load, a portion of the signals from a signal source will be reflected without being transmitted to the load side, in other words, the so-called reflection phenomenon will occur. As a result, noise which is produced due to the reflection phenomenon may be large and cannot be ignored. In this case, operation will be adversely affected, for example, a malfunction of the device will occur.
To solve these problems, a method for preventing reflection by matching the impedances between transmission line and LSI or device on the transmission end or the receiving end is generally used. FIG. 13 is a conceptual diagram showing the impedance matching principle of a transmission line. The impedance of a transmission line 1300 is Zo (Ω). A terminator 1301 for matching the impedance Zo of the transmission line 1300 is Rout (Ω). The terminator 1301 corresponding to the transmission line 1300 is connected in parallel relative to the transmission line 1300, as shown in FIG. 13. The value of terminator 1301 is adjusted so that the impedance Zo of transmission line 1300 and the impedance Rout of terminator 1301 are equal. In this case, the transmission line 1300 with characteristic impedance Zo is considered as equivalent to a line which extends infinitely. Theoretically, reflection does not occur.
FIG. 14 shows one example in which the impedances are matched based on the impedance matching principle in a signal receiving circuit employing a transmission line and a semiconductor device. As shown in FIG. 14, a terminator 1401 for impedance matching of a transmission line 1400 and a semiconductor receiving device 1405 is provided externally of the semiconductor receiving device 1405. The terminator 1401 has a resistance value equal to the impedance Zo of the transmission line 1400. In the method of FIG. 14, since the terminator is provided externally, the area of a printed circuit board where a signal receiving circuit 14 is wired will be large.
To reduce the area of a printed circuit board where a signal receiving circuit will be wired, a terminator is provided internally in FIG. 15. That is, FIG. 15 shows another example in which the impedances are matched based on the impedance matching principle in a signal receiving circuit employing a transmission line and a semiconductor device. As shown in FIG. 15, a terminator 1503 for impedance matching of a transmission line 1500 and a semiconductor receiving device 1505 is provided inside the semiconductor receiving device 1505. The impedance Zo of the transmission line 1500 and the impedance Rin of the terminator 1503 are equal. In the method, since the terminator is provided internally, the area of a printed circuit board where a signal receiving circuit is wired is smaller than the case shown in FIG. 14. However, since the terminator is provided internally in this method, its resistance value varies with voltage variation, temperature variation and the characteristics of the manufacturing process. For this reason, it is difficult to achieve impedance matching of the transmission line 1500.
To solve these problems, a technology in which a terminator of variable resistance is provided in a semiconductor receiving device 1605 is disclosed in Japanese unexamined patent application publication 2002-344300. FIG. 16 shows an example of this disclosure. Specifically, a plurality of sets of CMOS transistors and resistors are connected to a transmission line 1600 in parallel. The CMOS transistor and the resistor are connected in series in each set. The respective gate lines of the CMOS transistors are connected to a switch control portion. A detection portion for detecting the voltage and temperature characteristics is connected in the previous stage relative to the switch control portion. Operation of FIG. 16 will now be described. First, when the detection portion detects voltage variation or temperature variation, or additionally detects deviation of the resistance value due to the manufacturing process, the detection portion transmits a command signal in accordance with the situation to the switch control portion. The switch control portion determines a suitable resistance value of the variable resistor based on the command signal. Then, the switch control portion transmits ON/OFF switching signals for setting the suitable resistance to the CMOS transistors. Accordingly, the CMOS transistors turn ON/OFF according to the signals, thus, the terminator Rin is set to the suitable resistance value.
However, these impedance matching methods have the following problems.
In the method of FIG. 14, the terminator 1401 and the semiconductor device 1405 are branched and are connected through a small transmission line 1406 such as an input/output lead and an electrode wire. That is, the small transmission line 1406 and the small terminator 1401 appear to be connected to the transmission line 1400 in parallel. Accordingly, as shown in the impedance graph of FIG. 14, reduction of the impedance of the small transmission line 1406 appears. As a result, impedance mismatching occurs. Therefore, reflection cannot be suppressed.
In the method of FIG. 15, since the terminator 1503 is provided internally, its resistance value varies with voltage variation, temperature variation and the characteristics of the manufacturing process. For this reason, it is difficult to achieve impedance matching of the transmission line 1500. In addition, since there is no component for impedance matching of the small transmission line 1506, impedance mismatching occurs.
In Japanese unexamined patent application publication 2002-344300, impedance mismatching occurs caused by the impedance of the small transmission lines 1606 such as an input/output lead and an electrode wire. In addition, since a variable resistor is provided internally as the terminator, a mechanism for adjusting the resistance value of terminator such as a switch control portion, or a detection portion for detecting voltage and temperature characteristics is required. Accordingly, the circuit construction of the whole device is complicated. Generally, a variable resistor is formed by using CMOS transistors, as shown in FIG. 16. A CMOS transistor has a capacitive component between the drain and the gate, or between the gate and the source. For this reason, the frequency characteristics of the variable terminator is poor. Thus, its resistance value varies depending on the frequency of signal. Accordingly, as shown in an impedance graph of FIG. 16, the impedance corresponding to the CMOS transistors locally reduces, and the impedance Zo of the transmission line 1600 is not constant.